The conventional technique will be described with reference to the accompanying drawings.
FIG. 7 is a block diagram showing the structure of a conventional radiographic imaging apparatus which performs image correction. FIG. 8 is a circuit diagram schematically showing the structure of a conventional imaging unit used in the radiographic imaging apparatus. Note that radiation includes X-, α-, β-, and γ-rays, and the like.
As shown in FIG. 8, the imaging unit 701 used in the conventional radiographic imaging apparatus has an area sensor 803 in which pixels are arrayed in a two-dimensional matrix to perform matrix driving. The pixels comprise conversion elements 801a to 801i which convert radiation into charges directly and switching elements 802a to 802i. As for the conversion elements 801a to 801i, for example, p-i-n photodiodes or MIS sensors can be used. The p-i-n photodiodes or MIS sensors are preferably formed from amorphous silicon. As for the switching elements 802a to 802i, for example, thin-film transistors (TFTs) can be used. A bias voltage Vs from a power supply 804 is applied to the common electrodes of the conversion elements 801a to 801i of the respective pixels. The gate electrodes of the switching elements 802a to 802i of the respective pixels are connected to common gate lines Vgl to VgN. The common gate lines Vgl to VgN are connected to a gate driving device 805 including shift registers (not shown) and the like. The source electrodes of the switching elements 802a to 802i are connected to common data lines Sigl to SigM to output an image signal IS from a reading device 814 including a variable gain amplifier 811, analog multiplexer 812, A/D converter 813, and the like. Accordingly, the imaging unit 701 is formed from the area sensor 803, gate driving device 805, and reading device 814.
Image correction of the conventional radiographic imaging apparatus will be described with reference to FIG. 7. The image signal IS output from the imaging unit 701 is input to an offset correction unit 702. The offset correction unit 702 subtracts an offset correction signal OS based on offset data stored in an offset memory 703 in advance from the image signal IS to perform offset correction. The offset-corrected signal is input as a signal A to a gain correction unit 704. The gain correction unit 704 which corrects the gain characteristics of the respective pixels performs an arithmetic process such as division for the signal A and a gain correction signal GS based on gain correction data stored in a gain correction memory 705, to output the obtained result as a signal B to an output unit 706 such as a monitor (see patent references 1 and 2).
[Patent Reference 1] Japanese Patent Laid-Open No. 2003-244557
[Patent Reference 2] Japanese Patent Laid-Open No. 10-327317
While the conventional radiographic imaging apparatus has the offset correction unit 702 and gain correction unit 704, as described above, its gain correction signal corresponds to only a single mode.
As shown in FIGS. 9A and 9B, in the conventional radiographic imaging apparatus, the gain characteristics, i.e., input/output characteristics sometimes differ between moving image radiography in which radiography is performed continuously and still image radiography in which radiography is performed intermittently. In particular, according to experimental results, when the imaging unit 701 uses conversion elements formed from amorphous silicon, the difference in gain characteristics of the conversion elements between moving image radiography and still image radiography becomes obvious.
The difference in gain characteristics, i.e., input/output characteristics between moving image radiography and still image radiography will be described in detail. Note that in the specification, “gain characteristics” is used in a broad sense and means input/output characteristics, i.e., sensitivity characteristics.
A charge amount Q generated in the conversion element and input to each column of the reading device after radiation is transmitted through an object can generally be expressed for an incident photon (radiographic quantum) count P byQ(col,row)=Gs(col,row)×pγ(col,row) where Gs(col,row) is a gain or quantum efficiency specific to a pixel at each (col (column),row), and y (col,row) is an index (generally called gamma) specific to a pixel at each (col (column),row).
An analog output Vout which is output from an analog multiplexer of the reading device and corresponds to respective pixels can be expressed byVout(col,row)=Q(col,row)×Ga(col)/Cf(col)
where Ga(col) is the gain of an amplifier (not shown in FIG. 8) provided to each column of the reading device, and Cf(col) is the storage capacitance of the amplifier provided to each column of the reading device.
Accordingly, an analog output finally output from the analog multiplexer of the reading device can be expressed by the following approximate expression.Vout(col,row)=Gs(col,row)×pγ(col,row)×Ga(col)/Cf(col)
The present inventor has newly found that both Gs(col,row) and γ(col,row) depend on the following factors (1) to (5).    (1) the opening ratio, film thickness, film quality, and capacitance of the conversion element    (2) an electrical field applied to the conversion element    (3) if a phosphor is present, its film thickness and film quality (when the conversion element is a photoelectric conversion element)    (4) an environmental temperature    (5) the history of radiation (or light) irradiation or history of electrical field application
When the reading device is formed from an integrated circuit (IC) or the like, it should be noted that Ga(col), Cf(col), and the like, are nonuniform amounts which vary in accordance with the manufacturing process.
In a radiographic imaging apparatus, generally, not only a frame rate but also a dose to be applied to an object to obtain one image differs greatly between general radiography (still image) and fluorography (moving image). A charge amount generated in the conversion element differs greatly as well. Particularly, in an X-ray imaging apparatus using X-rays as radiation, in order to decrease a radiation dose of an object, a dose in fluorography (moving image) is sometimes decreased by at least one order of magnitude compared to that in general radiography (still image).
That is, when the gain Ga(col) or Cf(col) of the reading device is changed between general radiography (still image) and fluorography (moving image) in order to adjust an output range, variations in Ga or Cf in the reading device manufacturing process become a nonnegligible amount. Consequently, a correction error may occur in a correction method of the conventional radiographic imaging apparatus.
Furthermore, because the characteristics of the conversion element change depending on the above-described factors (1) to (5), for example, when an electrical field applied to the conversion element is changed in general radiography (still image) and fluorography (moving image), gain characteristics can change. Consequently, a correction error may occur in the correction method of the conventional radiographic imaging apparatus.
In addition, even when the history of radiation irradiation, history of electrical field application, environmental temperature, or the like differs between general radiography (still image) and fluorography (moving image), a correction error may occur in a correction method of the conventional radiographic imaging apparatus.
An example in which gain characteristics between general radiography (still image) and fluorography (moving image) change according to the history of electrical field application and that of radiation irradiation will be described below with reference to FIGS. 9A and 9B.
The X-ray dose to be applied to the conversion element and the frame rate differ between moving image radiography and still image radiography. The response speed of light when charges are trapped by the amorphous silicon, which is used in a conversion layer (i.e., a layer for converting radiation into charges directly) of the conversion element, can change between moving image radiography and still image radiography. This may cause a difference in gain characteristics of the conversion element as shown in FIGS. 9A and 9B.
In the case of still image radiography, sometimes the power supply of the imaging unit 701 is turned off for each radiographic operation. In this case, a bias applied to the conversion element 801 is also turned off. In the case of moving image radiography, radiographic operation is performed continuously while the power supply of the imaging unit 701 is kept on. In this case, a bias is continuously applied to the conversion element 801 during the radiographic operation. When the conversion element 801 of the imaging unit 701 is formed from amorphous silicon, such a difference in history of power supply application, i.e., a difference in history of bias application to the conversion element 801 may appear as a difference in gain characteristics of the conversion element.
The difference in gain characteristics as described above appears in several manners. For example, as shown in FIG. 9A, sometimes the sensitivity differs between still image radiography and moving image radiography. Also, as shown in FIG. 9B, sometimes gamma (γ) differs between still image radiography and moving image radiography. Although not shown in FIGS. 9A and 9B, sometimes gain characteristics 901a and 901b shown in FIG. 9A overlap gain characteristics 902a and 902b shown in FIG. 9B, and accordingly both the sensitivity and gamma differ between still image radiography and moving image radiography. These phenomena indicate that when the signal A obtained in moving image radiography is corrected by a gain correction signal suited to still image radiography, sometimes a correction error may occur.
Patent references 1 and 2 describe the functions of offset correction and gain correction. However, these references do not describe at all the correction error caused by a difference in gain characteristics according to the radiographic mode.
A correction error occurs, when the gain characteristics according to a frame rate, dose to radiograph one image, and usage method differ between moving image radiography and still image radiography, as the conventional X-ray imaging apparatus has only a gain correction signal which corresponds to a single mode, as described above. More specifically, when both moving image radiography and still image radiography are performed by the conventional X-ray imaging apparatus, gain correction is not performed appropriately, and sometimes the image quality is degraded.